1. Field of the Invention
This invention relates to a thermally responsive switch in which a thermally responsive bimetallic element including a shallow dish-shaped portion snaps in response to the ambient heat or due to self-heating to thereby make and break fixed and movable contacts, and more particularly to such a switch which has an operative temperature calibrating mechanism for setting the thermally responsive temperature by applying an external force to a convex side of the dish-shaped portion of the thermally responsive element when the element is in the contacts-closed position.
2. Description of the Prior Art
In the thermally responsive switch of the type described above, a thermally responsive element includes a shallow dish-shaped portion formed by way of drawing and is cantilever mounted in a cylindrical switch case or housing. A movable contact is mounted on a free end of the thermally responsive element in opposed relation to a fixed contact. Further, the switch is provided with a means for calibrating the operative temperature of the thermally responsive element. As one of such a calibrating means, a depressing member is provided in the housing. The member is depressed against the convex side of the dish-shaped portion of the thermally responsive element. Stress exerted on the convex side is adjusted by changing the intensity of pressure exerted by the depressing member, thereby calibrating the operative temperature of the thermally responsive element.
In the above-described construction, however, the stress exerted on the convex side by the depressing member is concentrated in the vicinity of a supported end of the thermally responsive element, which causes the supported end to suffer plastic deformation, thereby narrowing a range of operative temperature to be calibrated.
Furthermore, where a stud bolt is employed as the depressing member, the calibrated temperature changes in the course of use of the switch due to the loosening of the stud bolt. Another member such as a spring washer or double nut is employed to prevent the stud bolt from loosening, which renders the calibrating work difficult. Although the pressure against the dish-shaped portion of the thermally responsive element can also be gained by the bending deformation of the depressing member other than the stud bolt, spring back induced at a bent portion of the depressing member changes the calibrated operative temperature.